Fiber-optic current sensors (FOCS) commonly rely on the Faraday effect in fused silica fibers. The Faraday-effect varies with temperature. The Verdet constant V of fused silica fiber, which is a measure for the Faraday effect, changes according to (1/V) ∂V/∂T=7×10−5° C.−1, i.e. within a temperature range of operation of e.g. −40° to +80° C. the sensor signal varies within 0.84%. Many applications of FOCS require accuracy to within ±0.2% or ±0.1%, and therefore require measures for temperature compensation.
In EP 1107029, EP 1115000 and K. Bohnert, P. Gabus, J. Nehring, H. Brändle, “Temperature and Vibration Insensitive Fiber-Optic Current Sensor,” J. Lightwave Technol., 20(2), 267, (2002), a method is described for inherent temperature compensation of the Faraday effect in interferometric Sagnac and reflection-type fiber-optic current sensors. The method of inherent compensation eliminates the need of an extra temperature sensor, which is particularly important for current sensing at high electric potentials. This method exploits the temperature dependence of the fiber-optic retarder which generates the normally circular light waves propagating in the sensing fiber. For temperature compensation the retardation is set to a value which differs by a non-zero amount ε from the conventional 90°-retardation. The variation of the retardation with temperature, affects the scale factor of the sensor. At the properly chosen retardation, e.g. ε=10°, the influence of the retarder on the sensor sensitivity (normalized scale factor S) balances the variation of the Verdet constant with temperature.
In conventional systems, the retarder is initially prepared with an over-length, i.e., a retardation larger than the target retardation. The proper retardation is then approached by fine-tuning the retardation in a stepwise manner. After each fine-tuning step the retarder contribution to the temperature dependence of the sensor is measured. The measurement involves translating the retarder to a temperature controller, changing the retarder temperature within a certain range and monitoring the resulting effect on the sensor signal for a given current. The fine-tuning is continued until proper compensation is reached. Fine-tuning is done by heating the retarder in the arc or heater filament of a fiber splicer. The heating alters the retarder's linear birefringence and thus retardation, e.g. by diffusion of dopants out of the fiber core and/or by a change in fiber stress.